1. Field of the Invention
The present invention relates to a Doppler direction finder and, in particular but not exclusively, to a Doppler direction finder for use in a cellular telecommunications network station. The present invention also relates to a method of locating, for example, a mobile station using a Doppler direction finder.
2. Description of the Related Art
Doppler direction finders are known which are designed to determine the direction from which a signal has been received. However, these known Doppler direction finders are generally designed to work with received signals having a relatively simple modulation such as for example frequency modulation or amplitude modulation. The known Doppler direction finders may not be suitable for use with received signals having a relatively complicated modulation such as Gaussian minimum shift keying (GMSK). In particular, the known Doppler direction finders require an increased amount of time in order to determine the direction from which a signal has been received, if complex modulation is used. This can be a problem if the direction finder has to determine the directions from which a number of signals are simultaneously received. Further, the accuracy of the known Doppler direction finders may decrease with increased modulation complexity. With more complex modulation, the detection range, and accuracy of the Doppler direction finder is reduced.
In known cellular communication networks, a fixed base transceiver station (BTS) is arranged to transmit signals to and receive signals from mobile stations (MS), for example mobile telephones, which are located in the cell or cell sector served by that base transceiver station. For general communication traffic between the BTS and MS, information on the exact location of the mobile station within the cell or cell sector is not of importance provided that the quality of the communication between the BTS and MS is satisfactory. However, the US Authorities (FCC) have recently introduced a regulation that requires the location of a mobile station within a cell or cell sector to be pinpointed within 125 meters when that mobile station makes a call to one of the emergency services. Several methods have been proposed for locating the position of the mobile station within a cell or cell sector. One method uses the principle of time difference of arrival (TDOA). With the time difference of arrival method, a signal from one mobile station is received by a plurality of base transceiver stations. By an observing the difference in time required for a given signal from a given mobile station to arrive at the plurality of base transceiver stations, it is possible to determine the location of the mobile station. However, this method can not be used if the signal from a mobile station only can be received by a single base station. Additionally, there are difficulties associated with the provision of the required timing signal at each of the base transceiver stations in order to determine the time difference.
In an alternative method which has been proposed, timing advance information is used. Timing advance information is required in time division multiple access (TDMA) cellular networks. In time division multiple access cellular networks, a plurality of time slots is used. A frame is made up of a predetermined number of time slots. Each mobile station will be allocated a given time slot in each frame in which to communicate with a base transceiver station. However, when the mobile station is relatively far from the base transceiver station, the signal will take longer to travel to the base transceiver station as compared to when the mobile station is relatively close to the base transceiver station.
In order to process correctly the signals received from various mobile stations, the signal from each mobile station must be received within its allocated time slot at the base transceiver station. To ensure that the signals are received within the allocated time slots the base station will provide the mobile station with timing advance information. This information indicates when the mobile station should transmit its signal to the base transceiver station. Accordingly, when the mobile station is relatively far from the base transceiver station, the mobile station will send its signal to the base station earlier than if the mobile station is relatively close to the base station. The signal transmitted by the mobile station will therefore be received by the base transceiver station in its allocated time slot. The timing advance information is thus a measure of the distance between the mobile station and the base station. However, this system is not accurate if information from only one base station is available. Furthermore, this method does not provide any indication as to the direction from which a signal has been received. This method can be improved if timing advance information from two base stations is considered. However, there are again problems associated with the provision of the required timing signals.